There are many applications in which it is desirable to track or locate objects or people, such as: tracking athletes for training or providing event information in real time; tracking emergency services or military personnel in buildings and urban environments; tracking staff, patients, and equipment in hospitals and nursing homes; and tracking staff and equipment in industrial, hazardous or mining environments for safety and automation.
A wireless localisation system refers to any system that uses transmission of electromagnetic signals (e.g. radio frequency or microwave) to localise (estimate the location of) an object, in two dimensions or three dimensions. Mobile objects can be localised and/or tracked by attaching a signal-enabled tag to the object and using a set of fixed ‘anchor’ nodes in the area to be monitored. Inaccuracies in the location estimates arise due to (1) the properties of the radio propagation environment (e.g. multipath reflections and diffraction) and (2) limitations in the system hardware (e.g. lack of time/frequency synchronisation and propagation delays in hardware that are time varying). The latter issues are particularly severe in applications where the anchor nodes have wireless connections and must consist of low-cost hardware.
There are numerous systems for wireless localisation of objects or people. Optical, infra-red and ultrasonic localisation do not work through walls. Amongst radio localisation systems, the various techniques rely on measurement of received signal strength (RSS), time-of-arrival (TOA), and/or angle-of-arrival (AOA). It is well known that in difficult radio propagation environments, RSS techniques have poor accuracy. AOA techniques require expensive hardware to determine the direction of arrival, and may perform poorly in multipath environments where reflections arrive from many directions. A common TOA-based technique is satellite navigation (e.g. GPS); however, this is not possible in indoor environments or even in outdoor environments where the accuracy is significantly degraded by multipath signals (e.g. urban canyons).
The most common TOA-based localisation system uses receiving anchor nodes that are hard-wired (cabled) to the processing hardware (e.g. U.S. Pat. No. 6,831,603). This greatly simplifies the system as a common clock can be shared, eliminating the problem of frequency and time synchronisation. In some situations, such as where rapid installation is required or the region between receiving nodes is inaccessible or inappropriate for cable installation, cabled connections between anchor nodes are impractical.
Where there is wireless connection between anchor nodes, the frequency and time synchronisation problem is often handled by using two way (also known as round trip) localisation, and usually also by the use of a reference node (e.g. US Patent 2003/0092448). This approach transmits a signal from one node to another, followed immediately by a return signal. The time between receiving the forward message and transmitting the reverse message is often assumed to be constant, which is not the case in many practical systems.
Once the distance or ‘range’ between each mobile node and the anchor nodes has been determined, the location of the mobile nodes is estimated in a process known as ‘multilateration’. The most common technique uses a minimum mean squared error (MMSE) approach. With this technique, ‘bad’ range data can severely affect the estimated locations of the mobile nodes. Another technique with a different assumption on the error distribution is based on Projections onto Convex Sets (POCS); however, conventional POCS algorithms do not handle well the case where there is a large intersection region. As with MMSE, the POCS approach is susceptible to bad data due to effects such as multipath reflections, radio interference and fading phenomena.